In powerful radio galaxies, the active galactic nucleus generates two oppositely directed jets of plasma with a relativistic bulk velocity. These jets terminate and convert much of their bulk kinetic energy into particles and magnetic fields at regions where they impact with the ambient medium at the outer edge of the lobe. This working surface is identified with hotspots, which typically have a compact structure and a high surface brightness. The impact of the jet generates a reverse shock that propagates upstream into the jet, and is responsible for the conversion of the bulk kinetic energy into random particle energies and turbulence. The jet material flows through the hotspots and back towards the nucleus, spreading out to form the extended radio lobes (e.g. [1]}).
